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Unread 08-12-2020, 08:35 PM   #1
Sparkles
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Retiling fireplace hearth: are my methods sound?

Hi there,

I'm retiling the fireplace hearth in my 110 year old house using sheets of 13" x 11" x 3/8" mosaic stone tile (see last photo). I would like the the tile to be flush with the surrounding hardwood floor.
I removed the original tile and encountered a very uneven and somewhat crumbly concrete deck.

I plan to:
1) roto-hammer the high points of the concrete down to the appropriate height using a chisel bit (the high spots of the concrete won't accommodate the thickness of my tile);
2) remove the concrete crumbles and wet the remaining deck;
3) use Thinset and a screed board to bring the hearth deck up to the correct height and make it true with the floor;
4) install tile with a second layer of Thinset using a 1/4" x 1/4" square notch trowel;
5) grout with un-sanded grout.

Does this seem like a reasonable plan?

Do I need to allow the initial layer of thinset to cure before installing the tile?

And if I use a 1/4" x 1/4" trowel, the second thinset layer would be 1/8", right?
Which means I should make a screed board with 1/2" notches on each end? (tile thickness + thinset thickness = screed board notch depth)

Also, do I need to back-butter the mosaic sheets?

Lastly, there is a thin crack in the concrete...do I NEED to use a crack-isolation membrane or can I get away with just using fiberglass tape and Thinset forced into the crack? Or better yet, can I ignore the crack altogether since 1) no one will walk on the tile and 2) my excitement about this project < my desire to go play outside?

Thank you!
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Last edited by Sparkles; 08-12-2020 at 08:40 PM. Reason: typo
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Unread 08-12-2020, 10:04 PM   #2
Tool Guy - Kg
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Welcome to the forum, Sparkles!

1) That might be a bit rough for what you’ve got and cause unintentional damage. Does a screwdriver pressed into that bed dig in, or skate off? If the screwdriver sinks in a bit, you’ve got deck mud and a rub brick ($9 tool in the masonry section at the store) might be a better tool choice. It’s like sandpaper on steroids.

2) Okay.

3) Thinset mortar is sticky and doesn’t behave well when trying to screed it. It’s not meant for that application. In addition, thinset mortar shirks a fair amount, so thicker applications will shrink more than thinner applications....meaning that if you do use thinset mortar, allow it to dry for a couple days before proceeding with the tile. Otherwise, there are cement-based patching materials that are meant for this sort of thing.

4) Okay.

5) Okay, but that tile manufacture appears to have butted some of those tiles tight together. There really should be at least a 1/16” gap so that the grout can actually be applied between the tile pieces instead of barely filling the top most crack where it’s likely to flake off instead of being permanent.

6) See previous comments.

7) Yes, allow it to dry before installing the tile.

8) That would be true if you held the trowel at 90 degrees when applying the thinset mortar. In reality, it’s more likely to be barely 3/32”.

8a) ...so adjust your screed board as accurately as you desire.

9) That would likely be very messy. So, be sure to embed your tile in fresh mortar that hasn’t skinned over yet.

10) Using mesh tape and thinset mortar on a cracked substrate is sketchy. The mesh is more likely to help a bit than trying to squeeze some bonding mortar into what is likely a dusty crack (meaning that the dust is likely a bond breaker...so, no matter how awesome of a bonding material you get in there isn’t likely to join the two parts together). Will an anti fracture membrane work? That depends on how much horizontal and vertical movement is going on. Sorry, but this is pretty hard to assess from this side of the screen.

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Unread 08-13-2020, 11:30 PM   #3
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Thanks Tool Guy! I really appreciate your thorough reply.

Quote:
Originally Posted by Tool Guy - Kg
Does a screwdriver pressed into that bed dig in, or skate off?
A screwdriver definitely won't dig in. Some places are kind of crumbly, but the majority of the concrete is really hard. I initially tried using a masonry chisel and a 5 lbs sledge hammer, and that wasn't doing much of anything, no matter how hard I wailed on it.

Quote:
Thinset mortar is sticky and doesn’t behave well when trying to screed it. It’s not meant for that application...there are cement-based patching materials that are meant for this sort of thing.
Can you make any recommendations?

Quote:
That would be true if you held the trowel at 90 degrees when applying the thinset mortar. In reality, it’s more likely to be barely 3/32”.
I feel a bit dumb asking for clarification, but that means the thinset layer would really be too thin to care about when determining the screed board height? So, the screed board height would essentially equal the tile thickness...unless I want to get absurdly detail oriented?

Thanks again for the detailed reply! It's super helpful.
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Unread 08-21-2020, 10:17 PM   #4
Tool Guy - Kg
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Absurdly detail oriented?

Ah....that’s exactly what you want when remodeling.
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Unread 09-16-2020, 01:42 PM   #5
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Hey Tool Guy - Kg,

I still haven't tackled this project yet but I'm almost ready to do it.
You mentioned that I should use a cement-based product to level the hearth instead of thinset. Can you make any specific recommendations? For instance, when I go to the store, what should I look for on the product label so I know I'm buying the right thing?

Thanks!
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Unread 09-19-2020, 09:49 AM   #6
Tool Guy - Kg
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Go to the floor department. Read the label and specifically look for something that is sold in a powder material that you must mix up immediately before using (nothing “pre-mixed”) that says it is cement-based. Stay away from anything mentioning gypsum. Any reputable manufacturer will also have a good technical service department and it’s pretty easy to use your smartphone to look-up and call them while you’re looking.
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Unread 09-19-2020, 10:31 AM   #7
Davy
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I think deck mud will be the easiest thing to build up with. I would wipe the old concrete with a wet sponge and let it dry. Then apply a layer of thinset to the concrete as a bonder to bond the new deck mud down. You want the thinset to be wet when you dump the deck mud on it. You can spread the thinset with a 1/4 inch notch trowel or whatever you have handy.

About the easiest deck mud to find is Mapei's 4 to 1 at Lowes. It's a little too sticky in my opinion which makes it harder to work with than home made 5 to 1 deck mud but it will work and get plenty hard.

Notch your board like you said before. You didn't mention the thickness of your tiles but make the notches the tile thickness plus 1/16-1/8. Get the mud as flat as you can then slick it down with a steel trowel (lightly trowel over it). You don't want to have to butter up the tiles with thinset to over come humps in the mud. Take the time to get the mud flat and it'll save you grief when tiling.

Remember, you're mixing deck mud, not concrete. It's mixed with much less water. Check out the "liberry" and study up on deck mud (dry pack) before you mix it.
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Unread 09-23-2020, 06:34 PM   #8
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Tool Guy - Kg: Thanks for the specifics! That helps a lot.


Davy: Thank you so much for weighing in!
I originally planned to use deck mud but then I thought it would be too difficult for a newbie like me to do correctly. As a chemist, I make mixtures with carefully measured components, so the prospect of mixing up a batch of deck mud by eyeballing the amount of water added is a bit frightening. Having never mixed it before, I don't have an intuition for the proper consistency or moisture content--although I've heard the phrase "dry sandcastle material" tossed about. Do you really think it's doable for a weekend warrior?

Yes, the "liberry" is a very helpful resource! It taught me how to tile my bathroom and I think it looks pretty darn good.
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Unread 09-23-2020, 06:43 PM   #9
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Sure, you can do it. Just have everything ready to go before mixing the mud, including your notched board, a bucket of clean water and sponge for washing hand tools, etc.
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Unread 09-23-2020, 07:14 PM   #10
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Deck mud mixed at a 5:1 ratio is wet enough when you grab a handful, squeeze, no water comes out, but it sticks together...it's sort of like wet beach sand with enough cement in it to hold it together, is the right consistency. More cement makes it harder to deal with.

IT does pack tighter, and you want to do that, but basically, dump it, get it even, pack it, and add or scrape off some to get it the desired thickness and in plane. You don't have forever to work it, but if it's not really low humidity and windy, enough. It has a lot of strength in compression but not in bending or abrasion, but beneath tile, those characteristics are not important, only compression is. You're trying to make a proper, linear surface that can support tile.
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Unread 10-06-2020, 01:19 PM   #11
Sparkles
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Thanks so much Davy and jadnashua! I really appreciate your help.
I'll post pics if I ever get around to finishing this project.
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